(1) Field of the Invention
The present invention relates to a communication system, and more particularly to a communication system for performing WDM (Wavelength Division Multiplex) optical communications.
(2) Description of the Related Art
The optical communication network technology is a core technology for establishing the foundation of information communication networks. In view of growing demands in recent years for a wider range of more sophisticated information services, efforts are being made to promote the quick development of the optical communication network technology.
WDM, which is finding wide use in the field of recent optical communications, refers to a process of multiplexing different wavelengths of light to transmit a plurality of signals simultaneously over a single optical fiber.
WDM systems have a supervisory channel for supervising an optical signal, known as OSC (Optical Supervisory Channel). The supervisory channel is used to supervise WDM system nodes.
Problems of the conventional supervisory channel are that it is not flexible enough and suffers a lack of convenience because a fixed frequency is assigned to the supervisory channel between sections for WDM communications.
FIG. 10 of the accompanying drawings shows a conventional WDM system that is suffering problems. As shown in FIG. 10, a WDM terminal node 500 comprises optical transmission units 501, 502, a supervisory control unit 511, and a wavelength multiplexer 512.
The optical transmission unit 501 comprises an optical device 501a for emitting a main optical signal having a fixed wavelength λ1, and the optical transmission unit 502 comprises an optical device 502a for emitting a main optical signal having a fixed wavelength λ2. The optical transmission units 501, 502 are units that the user can install and replace as desired.
The supervisory control unit 511 emits an optical supervisory channel signal having a fixed wavelength λ3. Alternatively, the supervisory control unit 511 may be installed in another node, and the WDM terminal node 500 may receive an optical supervisory channel signal having a fixed wavelength λ3 from the supervisory control unit 511 installed in the other node. The wavelength multiplexer 512 multiplexes the optical signals emitted from the optical transmission units 501, 502 and the supervisory control unit 511 into a wavelength-multiplexed signal, and outputs the wavelength-multiplexed signal over a single optical fiber.
It is assumed that the user owns, in its inventory, only an optical transmission unit 503 comprising an optical device 503a for emitting a main optical signal having a fixed wavelength λ3. When the optical transmission unit 502 becomes faulty, however, the user cannot use the optical transmission unit 503 from the inventory to replace the optical transmission unit 502 because the optical signal of the wavelength λ3 has already been used by the supervisory control unit 511 or the optical supervisory channel signal of the fixed wavelength λ3 is being received from the other node. Consequently, the conventional WDM system suffers a lack of convenience due to the assignment of the fixed wavelength to the optical supervisory channel signal.
In recent years, networks are becoming larger in size and more complex in structure. In such larger and more complex networks, the conventional band available for OSC transmission fails to transmit sufficient control information, resulting in a reduction in the system efficiency.